Galling of casing and tubing threads has been a problem in the oil field for many years. It is particularly pronounced in the softer grades of quenched and tempered pipe. Galling is caused by two metal surfaces coming into "intimate" or "metallurgical" contact. When this happens metal is pressed against metal (without benefit of lubricant, oxide coating, or other protective film) with sufficient force that small areas atomically bond together. This can be viewed as spot welding on an atomic scale. When the metal surfaces are moved the metal is torn apart. Sometimes the rough metal in the torn areas will also damage the metal surfaces.
In order to prevent galling it is necessary to prevent the "intimate" contact of the surfaces.
Research has indicated that soft metals are more prone to galling than hard metals and that similar metal couples are more prone to galling than dissimilar metal couples.
Typical methods in use in the oil and gas industry involve the use of a lubricant which forms a layer between the surfaces. The lubricant is oil or grease. Sometimes particles of soft metals such as copper, lead, zinc, or tin are added. These particles serve two functions. Firstly, they act as a barrier preventing intimate contact of the surfaces. Secondly, under extreme pressure they deform and act as a lubricant between the surfaces.
Other approaches in the prior art have used conversion coatings on the metal surfaces in conjunction with lubricants. These conversion coatings are commonly referred to as "phosphate" or "black oxide" coatings or by their proprietary process name, e.g. "Parkerizing". They are characterized by the formation of a chemical compound (usually a phosphate or oxide) on the surface to be protected. During this process a portion of the original metal surface is dissolved. The conversion coatings prevent galling by (1) the interposition of a barrier layer (the oxide or phosphate) between the metal surfaces; (2) creating pockets of lubricant in the coating layer (due to the porous nature of the coating) which provide a more or less constant supply of lubricant to the surfaces under extreme pressure conditions; and (3) the creation of an irregular surface on the metal through the dissolution process.
Another method of preventing galling consists of "surface improvement treatments" of the surfaces to be protected. These treatments may be sandblasting or blasting with glass beads or shot. Liquid and vapor honing are also examples of treatments of this class. These treatments roughen the surface making many small dimples. These dimples serve as a lubricant reservoirs under extreme pressure conditions. A lubricant must of course be provided to fill such reservoirs.
Still a further method of preventing galling consists of applying a coating of soft metal such as tin or zinc to one of the surfaces to be protected. This technique is commonly also used with a lubricant. The metal coating acts as a lubricant of last resort under extreme pressure conditions. This method is described in U.S. Pat. No. 1,141,151 to F. N. Speller.
Sealing of casing and tubing connections is a problem in the oil and gas industry. It is difficult to run a string of pipe in a well and have a leak tight system. This is because the reliability of existing connections for leak resistance is not very high and because of the large number of connections which must be made in the string.
One solution for sealing is that used for API 8-round threads. A viscous material (such as API thread dope) is smeared on the threads before the connection is made up. The thread dope acts as a sealant for the joint. This system is not satisfactory at high pressures.
Another solution for sealing is that sometimes used for API buttress threads. The threads are coated with a soft metal such as zinc or tin and this metal coating serves to plug the gaps in the threads which could cause a leak.
A further solution for sealing is to use one or more metal to metal seals in conjunction with a threaded connection. Examples of this approach include Hydril Super EU, CS, and PH 6 connections, Atlas Bradford TC-4S and CQS connections, Mannesmann BDS and TDS connections and the VAM connection. These seals work because they form a tortuous path (on an atomistic scale) through which the gas molecules have difficulty in passing.
Still a further solution for sealing is the use of a ring or gasket of elastomeric or thermoplastic material in the body of the threads. An example of this would be the Teflon.RTM. ring in the Atlas Bradford TC-4S connection.
The present invention provides a method for preventing galling, provides an effective fluid seal, and provides porous lubricant reservoirs by covering one surface with a soft metal coating such as electroless copper or zinc.